This invention relates to the continuous casting of lead (i.e., including lead alloys) strip, and more particularly, the provision of a smooth, snag-resistant polyimide seal at the mouth of the mold cavity where freezing is initiated.
In the continuous casting of metals, and metal melt is introduced into the inlet of a casting nozzle having a chilled surface therein defining a mold cavity therethrough for solidifying the melt. The casting nozzle is thermally isolated from the melt source by a refractory material and melt passing through the refractory begins to solidify as a thin skin at the inlet of the chilled nozzle, which skin grows inwardly to form a solidified strip as it traverses the length of the nozzle. The casting nozzle is thermally isolated from the melt source (e.g., furnace, standpipe, etc.) by an insulating refractory material so that little if any heat is transmitted directly to the nozzle from the source. The inlet of the cavity adjacent the refractory is one of the most critical regions of the nozzle as it is the locus of the formation of the initial solid skin which permits pulling of the strip from the nozzle. The strength of the skin at the inlet plays a significant role in the rate at which the strip can be cast which, in turn, is a function of the metallurgical properties (e.g., tensile strength, etc.) of the metal itself and the thickness of the skin at the inlet. The combination of the metallurgical properties and thickness of the skin at the inlet determines the amount of pull the skin can withstand before rupturing. Skin rupture can cause the melt source to become "unplugged" and dump through the nozzle or otherwise create unacceptable defects on the cast strip. In the case of lead and other metals which are metallurgically weak, it is essential that there be no resistance to the initial skin's being freely pulled from the initial freezing zone else rupture readily occurs at relatively low casting rates. One source of this resistance to pulling is the junction between the nozzle inlet and the refractory inlet. Refractory materials heretofore used at the inlet to the nozzle do not interface well with the nozzle and have tended to limit lead casting rates to no more than about three and a half feet per minute (31/2 ft/min). In this regard, even with precision machining to achieve a butt seal, heat distortion and warpage can result in an imperfect seal between the nozzle and the insulator which permits minute flash to form in the interface which tears at the skin as it pulls away. Flashing can be eliminated by providing a slight land at the mouth of the inlet which bites into the insulator to provide a more positive seal, but this often causes the insulator, which is generally quite brittle, to chip and pit resulting in sites in the insulator where melt can solidify and become anchored to the insulator. Moreover, in the case of the asbestos-based materials such as Marinite.RTM. or Transite.RTM. microfibers from the insulator are often solidified within the skin while still anchored to the insulator. The flash, chipping, pitting and/or fibers all tend to snag, drag or otherwise locally hold the thin initial skin against movement and thereby severely limit the casting speed of metallurgically weak metals such as lead.
Accordingly, it is an object of the present invention to provide a snag-free and drag-free insulating seal at the casting nozzle inlet in apparatus for the continuous casting of lead whereby increased casting rates are possible without engendering ruptured skins. This and other objects and advantages of the invention will become more apparent from the description which follows.